Astronomy and Telescopes
The hunt for Earth-like planets, and perhaps extraterrestrial life, just got more precise, thanks to record-setting starlight measurements made possible by a National Institute of Standards and Technology (NIST) "astrocomb." NIST's custom-made frequency comb-which precisely measures frequencies, or colors, of light-ensures the precision of starlight analysis by an instrument called a spectrograph at the Hobby-Eberly Telescope in Texas. The project is a collaboration involving NIST, the University of Colorado Boulder (CU) and Pennsylvania State University, the primary partner in the telescope and spectrograph. 1)
The new comb apparatus for the first time provides the precision needed for discovering and characterizing planets orbiting M dwarf stars, which comprise 70 percent of the stars in the galaxy and are plentiful near Earth, the research team reported in Optica. 2)
Measurements of the periodic Doppler shift of a star using the spectroscopic RV (Radial Velocity) technique provide evidence of an unseen orbiting exoplanet and its minimum mass. 3) Complemented by photometric measurements of the transiting exoplanet, one can obtain the mass and radius (and density) of the exoplanet, which are critical parameters for classification and assessment of habitability. While the RV technique has been used extensively, the highest precision measurements have been limited to the visible region of the spectrum <700 nm, and not the infrared. 4) Thus, 70% of the stars in our galaxy—M-dwarfs, which primarily emit in the NIR (Near-Infrared)—have largely been outside the efficient spectral grasp of the current generation of proven RV instruments. 5) Due to their proximity, abundance, and small radius and mass, M-dwarfs are very attractive targets in the search for habitable-zone rocky planets with NIR spectroscopy. The habitable zone of an M-dwarf is close-in to the star, such that an orbiting Earth-mass planet within this zone induces an RV shift on the order of 1 m/s. 6) This is 10 times larger than the RV signature of Earth around the Sun, and significantly increases the detectability of such an Earth-mass planet. Stellar convection and magnetic activity (e.g., granulation and starspots) can obfuscate the small RV signature of terrestrial planets, but many of these noise sources are suppressed in the NIR compared to the optical. 7) High RV precision measurements of bright stars in the visible will benefit from complementary high RV precision NIR observations that better discriminate stellar activity from real planet signals.
While these motivations are well known, a combination of factors has limited the precision of even the best infrared RVs to 5–10 m/s over time scales of months. Low-noise silicon detector arrays are not efficient at wavelengths beyond ~900 nm, requiring infrared detectors and cryogenic instruments. Existing wavelength calibration sources in the infrared are not yet as effective as iodine cells and thorium–argon lamps in the visible. Fiber-fed spectrographs with the required intrinsic stability in the infrared are only now being built and tested. 9) 10)
We overcome many technological and analysis hurdles that have impeded NIR RV measurements from reaching the ~1 m/s level, and we introduce a complete suite of infrared spectroscopic tools and techniques that provide the necessary precision for discovering and characterizing planetary systems around M-dwarf stars (Figure 1).
We have developed a 30 GHz optical frequency comb spanning 700–1600 nm built on integrated electrooptic (EO) modulators and high-efficiency nanophotonic nonlinear waveguides to provide a robust calibrator for long-term operation at the telescope. This comb-based calibrator provides tailored light to the stable HPF (Habitable Zone Planet Finder) spectrograph, designed and built from the ground up for precision infrared RVs . The frequency comb and spectrograph have been installed at the 10 m HET (Hobby–Eberly Telescope), and we have demonstrated differential stellar RVs at 1.53 m/s (RMS) over months-long time scales, as well as shown that the comb-calibrated HPF can support RV precision as low as 6 cm/s.
To the best of our knowledge, this is the first time such RV precision has been realized in the NIR wavelength region beyond the spectral grasp of silicon detector arrays. This interdisciplinary result is an important step on the path to discovery and characterization of Earth-mass planets in the habitable zones of the nearest stars. The higher planet-to-star radius ratio in these M-dwarf systems will boost the transit signal by almost 2 orders of magnitude compared to Sun-like stars, making atmospheric studies of Earth-like planets feasible with current technology. Moreover, discovering and characterizing such targets is critical for the near-future detection of biomarkers in transiting terrestrial planetary atmospheres via spectroscopy with the James Webb Space Telescope.
Frequency Comb Instrumentation
Optical frequency combs,11) which consist of an equally spaced array of laser frequencies, have been recognized as a critical component in precision astronomical RV measurements aimed at identifying Earth-like exoplanets. 12) However, the combination of spectral coverage across hundreds of nanometers, line spacing of many tens of GHz (needed for modern high-resolution astronomical spectrometers), operational robustness in an astronomical observatory environment, and high up-time requirements are challenging to achieve. Multiple frequency comb approaches targeting astronomical spectrograph calibration have been explored, including mode filtering, EO generation, and microresonators. 13)
Our frequency comb is built around a combination of EO and integrated-photonic technologies to address the challenges of bandwidth, mode spacing, and robustness [Figure 2(a)]. In addition, we leverage recent advances with EO frequency combs to mitigate the impact of multiplicative microwave phase noise on the comb tooth linewidth and coherence.14) 15) The frequency comb begins with 1064 nm continuous wave (CW) light from a semiconductor laser that feeds waveguide EO modulators driven by a 30 GHz microwave source. This results in a comb of approximately 100 teeth spaced exactly by the microwave drive frequency. The CW laser, microwave source, and all other frequencies in the system are referenced to a GPS-disciplined clock that provides absolute traceability to the SI second. The initial comb is next passed through a 30 GHz Fabry–Perot cavity that acts to filter and reduce the impact of broad bandwidth electronic thermal noise. The frequency comb is then amplified in an ytterbium fiber amplifier, spectrally broadened and temporally compressed to a pulse width of 70 fs, and finally focused into a 25 mm long nonlinear silicon nitride (SiN) waveguide. 16)
The waveguide is 750 nm wide and 690 nm thick, which provides the combination of tight confinement and engineered dispersion to achieve a supercontinuum spectrum spanning 700–1600 nm with only 525 mW of incident average power (18 pJ of pulse energy). This low power reduces the thermal loading and aides the long-term operability. Following spectral generation, a combination of static and programmable amplitude filters  is used to tailor the spectral envelope. The output of each amplitude filtering stage is shown in Figure 2(b) along with high-resolution insets that show the resolved 30 GHz comb modes at 910 and 1210 nm. Optical heterodyne measurements between the 30 GHz comb and a frequency-stabilized reference laser confirm that the stability of the comb matches that of the GPS-disciplined clock with fractional uncertainty of 2 x 10-11 at 1 s, and <3 x 10-13 for time scales of one day and longer. Significantly, EO frequency combs could be employed for coverage from <600 nm to >2500 nm.
The entire frequency comb has been assembled on a 60 cm x 152 cm breadboard and installed at the HET at McDonald Observatory, together with the HPF spectrograph. The HPF is a vacuum-housed cross-dispersed echelle spectrograph with dimensions of approximately 1.5 m x 3 m and resolving power of R=λ/Δλ ~53, 000. The spectrograph is designed and optimized for stability, with its optics platform cryogenically cooled and temperature stabilized at the milliKelvin level. 17) 18) The 28 echelle orders cover the wavelength range 810–1280 nm, and the spectra are recorded on a 2048 x 2048 pixel Hawaii-2 (H2RG) mercury–cadmium–telluride (HgCdTe) infrared detector array that has a 1.7 µm long wavelength cutoff. The HPF entrance slit is fed by three optical fibers that can be simultaneously illuminated with star, sky, and calibration light to track the instrument drift.
Spectrograph calibration and radial velocity measurements
The frequency comb was integrated into the HPF calibration system at the end of February 2018 and has been operating continuously and autonomously since early May 2018. The light from the comb system is coupled to the HPF calibration source-selector system (see Kanodia et al.  for a layout) with a 50 µm diameter multimode fiber. This selector couples the light from any source (including the comb) into a 300 µm dynamic fiber agitator, which is coupled to one port of a 2 inch integrating sphere. We have shown  that the combination of temporal scrambling and an integrating sphere is effective at minimizing modal noise to deliver a constant illumination. The dynamic fiber agitator is a customizable commercial system from Giga Concept that applies torsional oscillations to the fiber (under compression) at a few Hz. An identical second fiber agitator is also used to couple the calibration light to the calibration fiber coupled to the telescope facility calibration unit.
On-sky stellar observations of stable M-dwarfs and stars with known orbiting planets have been employed to characterize the system's present RV precision. Figure 3(a) shows example stellar and comb spectra as recorded on the HgCdTe array, illustrating the HPF's spectral range and the uniformity of the comb across the 28 orders. Using the comb as a reference, a wavelength solution is derived for each order that is subsequently used to calibrate the corresponding stellar spectra. Benchmarking the full instrumental chain (from telescope to comb to detector), as well as spectral extraction, RV measurement algorithms, and corrections of barycentric RV, require on-sky observations of a star that is intrinsically stable or has a known RV signal. Barnard's star is a bright nearby star of spectral type ~M4. While its RVs show a low-amplitude signal attributed to an exoplanet,19) it is still among the most stable M stars known. Figure 3(b) shows the residual RVs of Barnard's star from 118 high signal-to-noise measurements over a three-month period. Observations were limited to 5 min exposures to prevent saturation of the NIR detector. The scatter of individual (5 min) RV measurements is 2.83 m/s, and when data within a ~1 hr HET observation window (or track) are binned to increase the signal to noise, the scatter is reduced to 1.53 m/s. This RV precision is unprecedented in the NIR, and approaches that of the best measurements for this star with visible-band spectrometers (e.g., 1.23 m/s using the HARPS instrument). 20)
Additional experiments allow us to probe the intrinsic calibration stability of the HPF spectrograph independent of stellar observations. To accomplish this, we take advantage of an auxiliary optical fiber that transmits the laser comb light to the primary focus of the telescope, where it is diffused and sent to the HPF spectrograph through both the "science" and "sky" fibers (Figure 4(a)). In this arrangement, two sets of comb calibration spectra are recorded by HPF and processed to provide RV wavelength solutions from both fiber channels. As the stable frequency comb is common to the fibers, the retrieved RVs will reveal the slow drift of the HPF in each channel. However, the difference of the two channels should ideally be zero within the uncertainty of the photon noise. As such, this measurement allows us to carefully probe noncommon-mode variations between the light paths from the two fibers to the detector array that would ultimately limit the RV precision. Such instabilities could arise from independent drifts in the optical fiber feeds, the spectrograph optics, systematics and noise in the HgCdTe detector array, or data reduction issues.
Measurements were taken over multiple nights, and the absolute drift of the HPF is recorded in both channels every 5 or 10 min, with the results shown in Figure 4(b). As seen in these data, the drift of the HPF was about 5–7 m/s during a 5–10 h measurement window, with larger daily excursions due to the liquid nitrogen fills. The difference of the RVs of the two channels is shown in Figure 4(c), revealing that the scatter of a single 10 min differential drift measurement is at the 20 cm/s level. We further see that if we bin multiple measurements, the RV precision improves approximately as the square root of total measurement time to as low as 6 cm/s at 300 min [Figure 4(d)]. Furthermore, we observe no statistically significant drift in the difference of the RVs retrieved from the two fibers over six days. These data support the assertion that the comb-calibrated HPF has the intrinsic stability to support infrared spectroscopy with precision below 10 cm/s. With the HET, this was a particularly harsh test since the calibration light splays across a significant fraction of the 30 arcmin focal plane—leading to fiber modal noise, which is known to be worse in the NIR than the optical. A fiber-based mode scrambler was used to mitigate modal noise, but it likely does not fully solve this issue. Our measured RV scatter is currently about 5 times greater than the photonlimited precision (3.7 cm/s in 10 min). Further improvements towards the photon-limited precision should be possible with algorithmic improvements, better characterization of the NIR detectors, and better modal scrambling.
In summary, we have introduced a new frequency comb platform in the NIR (700–1600 nm) that leverages the reliability of telecom EO modulators and efficient chip-integrated nonlinear waveguides to provide an ultrastable calibrator for the HPF spectrograph. Since May 2018, the frequency comb and HPF instruments have been running autonomously at the 10 m HET, enabling high-cadence RV measurements on one of the largest optical telescopes in the world. This combination of instrumentation forms a unique and powerful set of tools for exoplanet science focused on M-dwarfs. Our stellar RV measurements with scatter of 1.53 m/s are presently the most precise achieved in the NIR with HgCdTe detector arrays, and now approach the best RV measurements with more mature visible wavelength silicon detectors and associated technology, to the best of our knowledge. Within this context, the instrumentation and techniques we introduce represent a significant step towards the long-desired goal of RV spectroscopy in the NIR with precision at (and below) 1 m/s, as will be critical for the discovery and characterization of Earth-mass planets in the habitable zones of the nearest stars.
Finally, during the review of this paper, Ribas et al. (Ref. 19) announced the discovery of a 3.2 Earth-mass (RV amplitude = 1.2 m/s) exoplanet candidate orbiting Barnard's star with a period of 233 days. While our HPF RVs are not inconsistent with the orbit proposed by Ribas et al., our observations extending over 86 days coincided with the least dynamic region of the orbital phase curve, resulting in the flat time series shown in Figure 3(c). Nonetheless, it is significant that our first RVs from the combcalibrated HPF are at a level of precision that makes them relevant for state-of-the-art exoplanet astrophysics.
Funding: National Science Foundation (NSF) (AST-100667, AST-1126413, AST-1310875, AST-1310885); National Institute of Standards and Technology (NIST) (On-a-Chip Program); National Aeronautics and Space Administration (NASA) (NNX09AB34G); Pennsylvania State University and its Center for Exoplanets and Habitable Worlds; National Aeronautics and Space Administration (NASA) (NNX16A028H, Sagan Fellowship); Heising-Simons Foundation (2017-0494).
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).